This invention relates generally to the glass bending art and, more particularly, to press bending apparatus incorporating a novel hydraulic pressure release system for the press member actuator.
Bent glass sheets have become increasingly popular for use as glazing closures, particularly in sight openings for vehicles such as automobiles and the like. For such purposes, the glass sheets must be bent to precisely defined curvature dictated by the configuration and size of the openings in the overall design of the vehicle.
Generally, curved glass sheets intended for use in automobiles are tempered to increase their resistance to damage due to impact and to improve the breaking characteristics of the glass when broken so as to fragment into relatively small harmless particles as opposed to the large, jagged pieces resulting from the breakage of ordinary untempered glass.
One common method of producing curved, tempered sheets of glass is to heat flat sheets of glass to their softening temperature, press the softened sheets to the desired curvature between complemental shaping surfaces and then rapidly chill the bent sheets in a controlled manner to a temperature below the annealing range of glass. Generally, these operations are carried out successively while the sheets of glass are being advanced substantially continuously on a conveyor system along a horizontal path including a heating area, a bending area and a chilling or tempering area whereby the heat initially imparted to each sheet to bring it to the proper bending temperatue can also be utilized in the final heat treating or tempering operation.
The above referred to complemental shaping surfaces are formed on opposed upper and lower press members normally located above and below the horizontal path of movement of the glass sheets to receive the latter therebetween and are movable relatively toward and away from each other for pressing the sheets into the desired shape. Generally, an hydraulic actuator is employed for raising the lower press member upwardly to engage and lift a heated glass sheet off the conveyor system out of such horizontal path and press the same against the shaping surface of the opposed or upper press member.
In order to preclude glass sheet jam-up in the heating area, or anywhere along the conveyor system upstream of the press area in the event of hydraulic pressure failure when the lower press member is in an upper, extended position in the path of movement of oncoming sheets, a pressure relief valve is incorporated in the hydraulic system to dump or unload the pressure in the press actuator and allow the lower press member to descend into its lower, out-of-the-way position. However, these prior known pressure relief valves have not been entirely satisfactory. The ones applicant is familiar with commonly utilize spool valves and, because of infrequent usage and the close sliding tolerances imposed thereon, hydraulic fluid and other foreign matter that accumulates between the spool valve and its bore produces a "varnish" condition, causing the spool valve to stick when actuated and thereby abort its fail-safe function.